Combustion control system



July 29, 1941; Y -J K, MAWHA 2,251,019

COMBUSTION CONTROL SYSTEM Filed April 5,.1938 2 sheets -sheet 1 July '29, 1941 J. K. MAWHA COMBUSTION CONTROL SYSTEM Filed April 5, 1-938 2 Sheets-Sheet 2 'INVENTOR Patented July 29, 1941 NlTED TA PATENT 'oFFics COMBUSTION comaor. SYSTEM James K. Mawha, Pittsburgh, Pa., assignor to John M. Hopwood, Dormant, Pa.

Application April 5, 1938, Serial N0. 200,060 8 Claims. (Cl. 236-15) This invention relates to control systems forregulating the delivery-of air and fuel gas to furnaces and more particularly to a system for maintaining a proper mixture of air and fuel gas when the fuel gas consists of a mixture of different gases each having different qualities or heating values. g a

The control system embodying the invention is particularly adapted to furnaces which are fired with a fuel gas consisting of a mixture of 10 two gases having different qualities or heating values and where such mixture may vary from 100 percent of one of the constituent gases to 100 percent of the other or any proportion therebetween. The system, in one form of the in- 15 vention, may be utilized to so control the delivcry of a combustible mixture of air and fuel as to a furnace that a constant or uniform temperature is maintained in the furnace, and

to so regulate the quantity or rate of air which 39 ance with the quality of the fuel gas mixture, 80

in order to maintain a substantially constant degree of combustion efllciency.

The air required to burn a gaseous fuel at a given efliciency of combustion is proportional to Where a mixed 85 fuel gas is burned and the mixed fuel comprises say two different fuel gases, each having diflerthe heating value of the gas.

ent but constant heating values and diflerent but constant specific gravities, therewill be a definite relation between the volumetric heat- 40 ing value of the mixtureand the specific gravity thereof. Therefore the amount of air required for a given efiiciency of combustion of the mixed fuel gas will be proportional to the specific gravity of the mixture as well as to its volumetric heating vaiue. I

An object of this invention is the provision of a control system that will maintain the proper mixture of air and a mixed fuel gas and which will utilize changes in specific gravities of the 60 apparatus,.such as shown in Fig. 4 in place of the device of Fig. 6, to adiust the rate 01 air supply in response to changes in specific gravity mixture to modify the amount of air supplied so that a uniform efiiciency of combustion may I be maintained for all values of the heating quality of the mixed fuelgas.

Another object of this invention is the pro And a further object of the invention is the vision of a system for so controlling the delivery of air and mixed fuel gas to afurnace that such a ratio between air and gas will be maintained as will result in eflicient combustion'of the fuel provision of a system forso regulating the delivery of air and mixed fuel gas to a furnace that a substantially constant or uniform efilciency of combustion may be obtained, and for so modifying the rate of delivery of air and fuel gas as to compensate for variations in quality or heating value of the fuel ga 'mixture that the same efliciency of combustion is maintained for all proportions of the constituent of fuel gasesofthemixedfuelgas. J

Qther objects of the invention will in part be apparent and will in part be obvious from the folowing description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a more or -less diagrammatic view of a control system embodying a form, of the invention, as applied to. the regulation of the supply of air and a mixed fuel gas to a furnace 35 which is illustrated diagrammatically; and in section;

Fig. 2 is a view of a device which regulates the supply of fuel gas to the furnace;

Fig. 3 is a view of a device which regulates the supply of combustion air to the furnace;

Fig. 4 is a more or less schematic illustration of circuits and apparatus for so adjusting the fuel supply regulator in accordance with the temperature existing at some selected point in the furnace that the heat input to the furnace may be regulated to maintain a substantially constant temperature in the furnace, this arrangement except for a slight modification, be-.

ing also utilized to adiust the rates of air and fuel supply to compensate for variations in heating qualityo'f the mixed gas fuel;

Fig. 5 isv a view in front elevation of a rotating disc or screen embodied in the arrangement 7 shown in Fig. 4; f

48 Fig. 6 is a view of an electromagnetically operated reflectoror mirror embodied in the apparatus of Fig. 4:

Fig. 'l'i's a view partly in section, of a reflect ing mirror and pressure responsive mechanism for operating the same, whichjs utilized with or heating quality of the mixed gas fuel:

motor operated leak-off valve embodied in the control system of Fig. 1; and

Fig. 9 is a top plan view of the motor drive for the leak-oi! valve.

In the drawings, a control system embodying a form of the invention is, for purposes of illustration, shown as applied to'a furnace I having a series of burners 2 (only one of which is shown) that are connected to a header 3 to which a fuel gas is supplied by a supply pipe 4. The fuel gas delivered to the supply P p from sources not shown, consists of a mixture of a; plurality of different gases, having diflerentbut substantially constant heating values or B. t. 11-. content, and different but substantially constant specific gravities.

The fuel gas delivered to header 3 therefore may be considered as a mixed gas fuel and comprising for example, two gas fuels, such as natural gas and coke oven gas. These gases have substantially uniform but different heating values or B. t. u. content and substantially uniform but different specific gravities. The supply of these constituent gases may vary from one hundred percent of one to one hundred percent sired constant value (assuming'that no change in the heating quality of the gas fuel has occurred) requiring an adjustment in the amount of fuel deliveredto the furnace, an adjustment is made in the tension of spring 22 (the loading on diaphragm it) which will result in such an adjustment of damper l3 as is required to change the rate of fuel delivery in the direction required to restore the furnace temperature to the desired value.

The adjustment or change in the loading on diaphragm I3 is accomplished by means of a rack 23 which is connected to spring 22, and driven by a reversible electric motor 24. Motor 24 is under the control of a device 25 having a temperature responsive element 26 such as a thermo-couple disposed in the roof of the furnace or at some other convenient or desired poof the other. As may be shown later herein,

the control system will respond to variations in the proportions of these constituent gas fuels of the mixed fuel gas to effect the proper rate of air supply so as to maintain substantially unlform eiiiclency of combustion for all variations in quality of the mixed gas.

The air required for the combustion of the mixed fuel gas may, for example, be supplied by a forced draft fan or blower 6 which is driven by a motor I and delivered to the burners 2 through a conduit 8. The intake 9 of the fan is provided with a damper l3 and a Venturithroat I I I. For a given mixture of mixed fuel gas delivered to the burners, a regulator l2 adjusts damper ill in such fashion that air will be supplied to the burners at such a rate that a uniform standard of combustion efllciency will be maintained for all rates at which-the fuel gas is supplied to the burners. The control system embodies means for so modifying the operation of regulator I 2 that the rate of air supply will be adjusted to suit variations in the heating quality of the mixed fuel gas and thereby maintain the efiicien'cy of combustion uniform.

The supply of mixed fuel gas delivered to the header 3 may be controlled by a'valve or damper i3 located in supply pipe 4, which is operated by a regulator ii. For a given quality or heating value of the mixed fuel gas and a given furnace temperature, regulator i4 adjusts damper l3 in such manner that the pressure of the gas in the header 3 is maintained substantially constant. In Fig. L-regulator I4 is illustrated only schematically but is shown in a practical form in Fig. 2. As schematically illustrated, this regulator includes a housing is having a diaphragm 16 therein which is connected by a lever i1 and link it to damper it. header 3 is communicated to housing l5 by pipes l 9 and 26, an adjustable orifice 2! being disposed in pipe la. The function of this orifice will be described later herein.

The pressure of the gas in sure of gas in the header when the damper isin the proper position to effect suchpressure.

If the furnace temperature varies from a desition and a reflecting element 28 which is oscillated by a magnet coil 26" and armature R. Variations, in temperature of the thermo-couple are utilized to variably energize coil 26" to control the magnitude of the current output of an electrical system such as shown in Fig. 4 and embodied in device 25. This electrical system may be of the type-disclosed in the application of Smith and Specht, Serial No. 704,518, filed December 29, 1933, and assigned to John M. Hopwood. The current output of this system is utilized to energize a solenoid 21 which acts on a movable core or armature 23 that operates a movable member or pointer 23'having a contact member 30 at one end that moves between two contact members 3| and 32. Contact members 3| and 32 are stationary relative to each other but mounted for movement with or by the rack 23.- As shown, these contacts are carried by a block 33 secured to rack 23. Movable member 29 is biased to 'a neutral positlon'or midway position, for a given current output of the system of Fig. 4, by means of an adjustable spring 34.

If the temperature of the furnace is rising, the current output of this system increases so that contact member 3| is engaged by contact member 30 causing motor 24 to be connected to a supply source and turn in such a direction as to move rack 23 upwardly to decrease the tension in spring 22 and thereby effect such an adjustment of regulator l4 that it will operate to maintain a lower pressure in header 3. If the temperature of the furnace is decreasing below the desired value, then movable contact member 30 engages contact member 32 causing the motor to operate in the reverse-direction to increase the tension in spring 22. This increased tension increases the loading on regulator 14 so that it will operate to maintain a higher regulated fuel gas pressure in header 3 and a higher rateof combustion in the furnace. By changing the loading on regulator H in accordance with the temperature in the furnace, this regulator will so increase or decrease the regulated pressure of the fuel gas that the desired temperature is maintained.

The adjustments in the loading on regulator H are made in incremental steps to avoid overadjustment with consequent over-adjustment in the pressure of the mixed fuel gas in header 3. These step-by-step adjustments of regulator M are made because as motor 24 is operated in one direction or the other, the contact members 3| and 32 being carried or moved by rack 23, break contact with the movable contact member 33 Therefore, a series of interrupted operations of the motor will occur ineither direction until the loading of the diaphragm of regulator l4 has been adjusted to such a value that this'regulator will maintain the proper rate of fuel delivery to the furnace for the particular amount of heatinpu-t required.

The drive between motor 24 and rack 23, and the control unit for the motor are indicated more fully in Figs. 8 and 9. A gear reducer 36 is disposed between the motor'and a pinion 31 that meshes with the rack and thereduction is one which allows the motor to run at high speed or tomake many revolutions while moving the rack slowly or a short distance. The reversing controller for the motor is illustrated schematically at 38 and maybe of any well known type.

As stated previously herein, regulator l2 re.- sponds to the rate of air flowv through the Venturi throat H and attempts to'maintain the air flow rate constant. This regulator also responds; to

a pressure in or to a pressure which is a function of the pressure in the mixed fuel gas header 3 so that the rate of air supply may be caused to be in direct proportion to the rate of supply of fuel to the furnace. As illustrated, regulators l2 and I4 are so connected to gas header3 that they are both subjected to the same magnitude or value of mixed fuel gas pressure.

Regulator l2, as diagrammatically illustrated,

of the air supply regulator to compensate for changes in the heating quality of the mixed fuel gas, an. adjustable leak-01f 49 is provided which operates to change the magnitude of the-fuel gas. pressure acting on regulators l2-and' l4.

' This adjustable leak-off includesa pipe 50 which ably, into the furnace where theescaping gas may be burned. In order to change thefuel gas 'pressure acting on-regulators l2 and l4,- to.com-

pensate for changes in the heating quality of the mixed fuel gas, valve 52 is adjusted If this valve is closed, the pressure drop across adjustable orifice 2| will be zeroso thatthe pressure operates damper 10 directly, whereas in a pracair cylinder 39, which in turn is directly coupled to the damper (see Fig. 3).

Referring to Fig. 1, itwill be seen that regumounted on a lever arm 42 and an inverted float 43 which is mounted on a lever 'arm 44,-'and these arms in turn are connected at their adjacent ends by a link 45 to damper l0. These floats work in a liquid seal, of water or oil which is contained in a tank 46. The interior of float 4| is connected by a pipe 41 to the Venturi throat ll so that itis subjected to the suction created at this point. The interior of float 43 is connected by a pipe 48 to pipes l9 and 20 so that it is subjected. to the same magnitude of fuel gas pressure that regulator I4 is. Thus floats 4| and 43 will oppose each other in their action on damper l0 as one float is under pressure and the other under suction. With this form of regulator, it will be seen that when once adjusted to provide the delivery of the proper amount of air to the burners for a given quality and pressure of gas in header 3, the rate of air supply will change with and in proportion to changes in the rate of fuel supply.

If the quality of the mixed fuel gas changes, as where the proportions of constituents of the mixed gas changes, the rate of air supply must be changed to maintain the efiiciency of combustion at the desired standard. If the heating value of the gas is decreasing because of a larger proportion of coke oven gas, for example, than of natural gas in the mixed fuel gas, then the rate of air supply must be decreased in propor-.

the mixed fuel gas, the rate of air supply mustbe increased. I c

To provide for the adjustment in the operation lator l2 comprises an inverted float which isations in the heating quality of the mixed fuel acting on float 43 of regulator 12 and on diaphragm IS on regulator I4 will b eequal to the pressure of the gas in header 3. As valve is opened, mixed fuel gas flows through theleakoff line causing the pressure drop across orifice 2| to rise so that the pressure acting on float 43 of regulator l2 and on regulator l4 will-be lower than the pressure 'in header 3'. The more valve 52 is opened the lower this pressure will be, be

cause the drop across the orifice 2| will increase with the increased flow of gas through leak-of! line 50'.

The lower the heat value of the mixed fuel gas, the more valve 52' is opened, because less air is required to maintain the eflic.ency of combustion up tothe desired standard; conversely,

the greater the heat value of the fuel gas becomes, or the more the heating value ofthis mixed gas approaches the heatingvalue, of the fuel gas-constituent having'the highest heating value, the more -valve 52 will'beclosed, as more air is required to maintain the desired combustion efliciency.

Valve 52 is adjusted in accordance with varigas delivered to, header 3 and since as stated previously herein, the specific gravity of the mixed gas is a measure of the heating value thereof, means 54 are provided for measuring the'chan'ges in specific gravity of the gas and causing valve.

' 52 to be adjusted in accordance with such me'asurements. Valve 52 maybe operated by any suitable means for example, by the rack and motor drive described in connection with the regulator 14 as shown in Figs. 1 and 7. The adjustments of valve 52 either towards open or closed position, are made in incremental steps for reasons already stated.

The control of the motor andrack drive for valve 52 includes an electrical system such as shown in Fig. 4, and a specific gravity measuring device 55 forming a part of means 54 which causes the output current of this system to vary with variations in the specific gravity of the mixed fuel gas in header 3.

Device 55 comprises two pipes 56 and 51 which are relatively tall and which are connected at their lower ends tothe air supply conduit 8 and gas header 3 by pipes 58 and 59, respectively, in which needle valves 60 and are disposed. The upper ends of these pipes are" open to the atmosphere and the lower ends are connected to a casing 62 which is dividedinto chambers 63 and 64 by a bellows or flexible diaphragm 65.. When 'these pipes are filled with gas, and air respecgas and these columns are connected totheilovver .l and upper chambers 63 and 64, respectively. The

needle valves 60 and GI are adjusted to allow just enough gas and air to flow into. pipes 56 and 51 to just keep them full, the excess merely bubbling over the tops thereof. Since the weight of the air column will be sub- ,stantially constant, it will therefore maintain a substantially constant pressure in chamber 63 and since the weight of the gas column will vary with changes in the quality of the mixed fuel gas, the diaphragm or bellows 65 will be flexed or'displaced inproportion to the diiIerence between the-weights of these columns. The bel- :lows and diaphragm are utilized to operate an element 61 (see Fig. 7) which causes the output current'of the electrical system, such as shown 'in Fig. 4, tovary by and in accordance with the variations in the specific gravity of the mixed fuel gas supplied to header 3 and, therefore, in accordance with the variations in the volumetric heat content or heating quality thereof. Element 6'! comprises a reflecting mirror which is *mounted at the end of a leverv 68supported on a stem 68 carried by bellows 65 and which extends through an opening in the top of casing 82. This opening is sealed with an expansible. bellows I0. Lever 68 operates against a fulcrum H and isprovided with a light spring I2 which yieldingly opposes the rocking force supplied to it by the bellows.

The position of the reflecting mirror, being a A- practical form of apparatus for operating damper l3 which controls the pressure of the fuel gas in header 3 is illustrated in Fig. 2. Instead of utilizing diaphragm I6 of regulator I 4 to operate directly the damper as shown diagrammatically in Fig. 1, this regulator may be utilized to actuate a pilot valve 14 of the air or coupled to the fuel supply damper. Thus small movements of diaphragm l6 may be utilized -to obtain large, as well as small, adjustments of the damper. Motor39comprises a cylinder 15 having a piston therein from which a piston rod 16 extends, through one end of the cylinder. The piston rod .carries a frame 11 which is connected to damper l3. The pilot valve 14 controls the admission of compressed air or other motive fluid to either side of the piston and allows such fluicf to exhaust from either side, motive fluid exhausting from one side when it is admitted to the other. If the pressure on diaphragm IS increases, the pilot valve is shifted to admit fluid to the cylinder on the bottom side of the piston so that frame 11 is moved upwardly to shift damper l3 towards closed position to restore the pressure in header 3 to the desired value. If the pressure is decreasing the diaphragm 7 l6 moves pilot valve 14 in the opposite direction to admit motive fluid to the top of the cylinder and cause the piston and frame to move downwardly and shift the damper l3 towards open position to increase the pressure in the header towards the desired value. cut-off mechanism that is operated by the regulator frame to return the pilot valve to ofi position when the frame has traveled a predetermined distance, depending on the amount the valve was opened. This mechanism comprises a cam bar which connects the cam bar to the bell crank, and all connected as shown and which cooperate to return thepilot .valve to oil position as just' stated. The valve is initially operated to one or the other of its on positions'by a lever 83 which carries tife bell crank 8| at one end and is connected at its other end to diaphragm I8 by link I8. Thus, step-by-step movement or adjustment of the damper is obtained. Since the construction and operation of this motor is well known in the art and illustrated in prior patents, of which Patent No. 1,931,906, dated Oct. 24, 1933, is an example, it is believed unnecessary to further describe this device herein.

In Fig. 3, a practical form of regulator is shown for operating the air supply regulating damper ill. Instead of operating the damper di-' rectly by the dual float regulator l2, this float regulator is employed to operate an escapement valve 85 which controls the delivery of an operating medium such as compressed air to a spring loaded bellows 86 that operates a pilot valve 14 of motor 39, like motor 39 shown in Fig. 2. When the heat content of the mixed fuel gas is decreasing so that a higher rate of gas supply is needed for combustion, escapement valve 85 is so operated by the dual float regulator as to deliver decreasing pressures to bellows 86 causing the frame to move downwardly step by step to shift the air control damper towards open position. If the heat content of the mixed fuel gas is increasing, increasing pressure impulses are delivered by valve 85 to bellows 86 to cause frame 11 to move upwardly step by step and shift the damper towards closed position and decrease the rate of air supply.

The construction and operation of this escapemerit valve is well known in the art, being shown hydraulic-motor, which in turn is directly The fluid motor 39 includes a in Patent'No. 1,931,906 granted October 24, 1933, above mentioned as'well, as in other prior art patents, and needs no further description herein.

The electrical system shown in Fig. 4 and which is employed in the temperature measuring gage 25 and in the specific gravity gage 54 includes a grid controlled glow discharge tube 81, having a plate 88, a grid 89 and a filament 98, an amplifying tube 9| having a plate 92, a grid 93, and filament 94, and a photoelectric cell also connected in circuit with each other that the photoelectric cell controls the fiow of current in the plate circuit of tube 9i and this tube in turn controls the grid of tube 8'| and the flow of current in its plate circuit.

The photoelectric cell 95 is caused to pass current at such times that the current output of device 81 will be in proportion to the temperature of the furnace-or in proportion to the specific gravity of the mixed fuel gas. The photoelectric cell is controlled by a rotating screen or disc 96.

having slots 96' therein and by a beam of light 91 which is reflected from mirror 61 or 26 and 'oscillated along a diameter of the disc, say the The power for operating the photo-cell 95 and amplifier tube 9| may be supplied by means of a power pack I08 comprising a transformer having a primary winding Ifll, a secondary winding I02 to which a thermionic rectifier I03 is connected, and low voltage windings I04 and I05 for supplyfollower lever 88, bell crank 8| and a link 82 [5 ing the filaments of the rectifier and amplifier I a e 2,251,019

tubes, respectively. The output of the rectifier is passed through a potentiometer B8B and a filter comprising a choke. coil 10! and condenser I08. The current delivered through potentiometer I08 is unidirectional and flows in the direction indicated by the arrow I09. The voltage drop across the potentiometer provides the direct current "The energizationor the photoelectric cell is effected by the light beam which is reflected from mirror 61 to the rotating screen N in such fashion that for each position of the light beam in its travel between limits A and B on radius -1! of the screen, the glow' tube will break down and pass current at a diflerent but definite point in voltages supplied to photoelectric cell andto the plate and grid circuits of the'amplifying tube, and the grid of the glow tube 81.

The plate or anode 92 of tube 9| is connected through an impedance Iii! to positive terminal ill of the potentiometer as is the anode of the to section N9 of potentiometer I06. The sliding contact is so adjusted that the grid bias is normally such that no current will flow in the plate circuit of tube 87 when the plate circuit voltage is applied. If the photoelectric cell 95 is energized, in a manner to be hereinafter described, to

. cause the amplifying tube to pass current, then a rapidly rising or steep wave front voltage will be developed in impedance lit which will render the grid'of the glow tube 8'! sufliciently positive to make the tube pass current at any instant during the application of the positive cycles of the voltage applied to its plate circuit.

The voltage for the plate circuit of the tube 87 is supplied by a secondary winding 826 of a transformer I25 in which circuit solenoid 2? is connected. A milliammeter I22 calibrated in terms of temperature, or in terms of specific gravity of the mixed. fuel gas, and a recording instrument l23 suitably calibrated may be included in the plate circuit. An adjustable resistor i2 5 connected in the plate circuit may be utilized to limit the current output of the tube to a predetermined maximum value, and a condenser i 25 may be connected across the instrument and I solenoid to smoothen thelcurrent flow in this cirsynchronous'motor N3 and slots 96' are spaced,

180 electrical degreesapart. Since. motor I is the positive half cycle-of the alternating current plate voltage; and these break-down points willcorrespond to different values offurnace temperature or of specific gravity of the mixed fuel a gas.

In order. that the time of impingement oi the light beam on the photoelectric cell may be caused tooccur only during the times that the plate voltage of tube 81 is positive, disc 86. is driven by 0.

operating in synchronism with the plate voltage, these slots are soproportioned that every other slot sweeps across radius O-Y during the time that the plate voltage is positive. In other words, if 60 cycle alternating current voltage is applied to the plate circuit of tube 81, then disc 98 should cult. Transformer iZi includes a low voltage winding $26 which supplies filament 9t, and a winding E2? to which light 98 is connected. The neutral point of winding lid is connected by a conductor I28- to the terminal of impedance lid which is connected to plate 92 of tube 95 and this conductor in turn is grounded at l29. Since the neutral point of winding m5 is connected by a conductor I38 to potentiometer tilt, whereby point itl of the potentiometer becomes in efiect the zero potential point.

Since alternating current voltage is connected to the plate circuit of tube 8?, this tube will pass current only during the positive half cycles of this voltage providedthe grid bias voltage is of the proper magnitude and polarity.

As previously stated herein, the grid bias on tube 8'8 is normally adjusted so that no current will flow in the plate circuit of this tube. This adjustment is made While the photo cell 55 is'deenergized, by moving sliding contact lit towards neutral point it! until the glow tube is just at the critical break-down point, that is, just at the verge of. passing current. The slider is then moved further towards the neutral point until the negative bias has been increased a predetermined amount, say'ten'percent or more if necessary. 1

travel at such speed that each slot 96"101' example, would sweep across radius 0-Y (the path 0! travel of the light beam) in $420 of a second.

If the light beam is impinging on disc 88 at a point where it will shine through the outer end of slots 96' (assuming that disc 96 is rotating in the direction of arrow ltd) then the photoelectric cell 95 will be momentarily energized andcause current to flow in the plate circuit of tube 9!. This flow of current induces an abruptly rising voltage in impedance Ht that neutralizes the normal negative bias voltage on the grid of tube 87 and imposes thereon instead a positive bias voltage of sufficient magnitude that the glow tube will break down at the beginning of the positive half wave of the plate voltage and pass' current for the full half cycle. When the light beam is impinging on the disc 'at a. point where it will pass through the outer end of the slots at, the glow tube will pass maximum current. If the light beam impinges on the disc at any point mum when the light beam shines. through the inner ends of the slot.

If the disc is driven by a consequent pole type alternatingcurrent motor which, in efiect, is an induction motor capable of operating at synchronous speed, as many slots as the motor has poles are required. Thus, ifa four pole motor is employed, the disc will have four slots as shown,

and these slots will be spaced 180 electrical de grees apart so that when'the motor is started and brought up to synchronous speed, there will always be a pair of slots disposed 180 electrical degrees apart through which the light beam may; a shine in the proper phaserelation toithe plate voltage of tube 8i. 'Therefore, the steep wave front or abruptly rising voltages impressed on the grid of device 8'! will always be in phase with and occur at some time during the positive half cycles of the alternating voltage of this device.

If a salient pole motor is utilized and provided with direct current field windings, only half the number of slots requiredpfor a conse quent pole motor would be necessary. These slots, of course, would be 186v electrical degrees apart and separated by solid sections of disc, each embracing an angle of electrical degrees.

, means responsive to By giving the slots in disc 95 a shape corresponding to the graph characteristics of the variable being measured or controlled, for example, the temperature. of the furnace, or the specific gravity of the mixed fuel gas, the output current of the glow discharge device will have the same graph characteristics as the variable. Thus, if it is desired that there shall be a linear relation between the pressure acting on the bellows of the specific gravity gage, or the temperature to which thermo couple 2c is subjected, and the output current of the glow discharge devices associated with these gages, the leading edges of slots 96 in the disc may be given shapes that accord with where p is the polar radius; R is the radius of the circle which is tangent to the inner ends of these slots; RI is the radius of the circle which is tangent to the other ends of these slots and is the angle in electrical degrees.

From this equation it will be apparent that the current output of the glow discharge devices used with the temperature and specific gravity gages may, for example, be caused to vary either as the square, cube, square root or any other exponential function of the specific gravity of the mixed fuel gas or-of the temperature of the furnace as measured by the thermo-couple.

While but one form of the invention has been shown and described herein, it will be apparent to those skilled in this particular art that various modifications and changes may be made without departing either from the spirit or the scope of the invention. It is desired therefore that only such limitations shall be placed on the invention as are imposed by the prior art and the appended claims.

What I claim as new and desire to secure .by Letters Patent is:

1. In a control system for furnaces, a source of supply of fuel gas comprising a mixture of gases having different B. t. u. values, a regulator adjusted to maintain the rate at which such mixture is delivered to the furnace substantially constant for a given mixture of such gases, means responsive to a departure in furnace temperature from a predetermined value for so changing the adjustment of said regulator as to effect such a change in the rate at which the fuel gas is delivered to the furnace as to restore the furnace temperature to said predetermined value, asource of air supply for the furnace, means responsive to the rate of delivery of air and fuel gas to the furnace for maintaining a substantially constant ratio between such rates of delivery for a given mixture of fuel gas, and means responsive to the relative weights of the air and fuel gas delivered to the furnace and acting upon said fuel supply regulator and said fuel-air ratio responsive means for automatically modifying the fuel gas and air ratio in such direction as to maintain efficient combustion in the furnace.

2. In a control system for furnaces fired with a mixed fuel gas, the constituent gases of which have different volumetric heating values and different but substantially constant specific gravities, and provided with a source of air supply, a regulator having means for adjusting it to control the mixed fuel gas at a substantially constant delivery rate for a given demand for heat, the rates at which the mixed fuel gas and air are delivered to the furnace for efficiency of combustion substantially constant for a given heating quality of the mixed fuel gas, and means responsive to a change in the specific gravities of the air and the mixed fuel gas and acting upon said mixed fuel gas regulator and the air supply adjusting means to so readjust the rates of delivery of air and gas that the efficiency of combustion is not substantially altered.

3. In a control system for furnaces fired with a mixed fuel gas, the constituent gases of which have different volumetric heating values and different but substantially constant specific gravities, and provided with a source of air supply, a regulator having means for adjusting it to control the mixed fuel gas at a substantially constant delivery rate for a given demand for heat, means responsive to the rates at which the mixed fuel gas and air are delivered to the furnace for so adjusting the air supply as to maintain the efficiency of combustion substantially constant for a given-heating quality of the mixed fuel gas, means responsive to a change in the specific gravities of the air and the mixed fuel gas and acting upon the gas regulator and the air supply control means to so readjust the rates of delivery of air and gas that the efficiency of combustion is not substantially altered, and means responsive to a change in furnace temperature, as affected by a change in the demand for heat, for modifying the adjustment of the fuel supply regulator and causing it to regulate the fuel supply at a rate which will restore the furnace temperature to a value corresponding to the demand for heat.

4. In a control system for furnaces fired with a mixed fuel gas, the constituent gases of which have different volumetric heating values and dif-- ferent but substantially constant specific gravities, and provided with a source of air supply, a regulator having means for adjusting it to control the mixed fuel gas at a substantially constant delivery rate for a given demand for heat, means responsive to the rates at which the mixed fuel gas and air are delivered to the furnace forso adjusting the air supply as to maintain the efficiency of combustion substantially constant for a given heating quality of the mixed fuel gas, and means responsive to the difference between the specific gravity of the mixed fuel gas and the air delivered to the furnace for effecting such adjustments 'in the operation of the gas rate regulator and the air-gas ratio regulator that the rates of air and fuel delivery are so modified that the efficiency of combustion is maintained at a substantially constant value for substantially all values of heating quality of the mixed fuel gas.

5. In a control system for furnaces fired with a. mixed fuel gas, the constituent gases of which have different volumetric heating values and different but substantially constant specific gravities and provided with a source of air supply, a regulator having means for adjusting it to control the mixed fuel gas at a substantially constant delivery rate for a given demand for heat, means responsive-to the rates at which the mixed fuel gas and air are delivered to the furnace for so adjusting the air supply as to maintain the efficiency of combustion substantially constant for a given heating quality of the mixed fuel gas, and means responsive to the difference between the specific gravities of the mixed fuel gas and the air for effecting such adjustments in the operation of said gas and gas-air ratio regulators that of said mixed gas fuel.

the rates of air and fuel delivery will be so modified that the efficiency of combustion is maintained at a substantially constant value for substantially all values of heating quality of the mixed fuel gas, and means responsive to-changes in furnace temperature for modifying the adjustment of the fuel supply regulator and causing it to regulate the fuel gas delivery rate at a value a pressure responsive control elementfor controlling -the regulator, a regulator for operating an element controlling the air supply rate and having a pressure responsive element connected with "the air supply' and operated in accordance with the rate of air delivery to the furnace, and a pressure element actuated by fuel gas pressure, both said air and gas responsive elements operating to. jointly control the air supply regulator to cause the air supply rate to vary by and in accordance with the gas supply rate for a given heating quality of the mixed gas fuel, a pipe connected to the fuel gas supply header, an orifice in said pipe, a second pipe connected through said orifice to said first mentioned pipe and to the pressure element of the fuel supply regulator and a pipe connected from said second pipe to the gas pressure responsive element of connected to the fuel gas supply header, an

orifice in said pipe, a second pipe connected through said orifice tosaid first inentioned pipe and to the pressure element of he fuel supply regulator and a pipe connected from said second pipe" to the gas pressure responsive element of the air supply regulator, a leak-oflf line connected to said second pipe for varying the pressures acting on the gas pressure responsive elements of said regulators, means for adjusting the rate of leak-oil to compensate for changes in the heating quality of the mixed gas so that the rates of air and fuel supply will be properly proportioned for any value of heating quality of said mixed gas fuel, and means responsive to changes in furnace temperature and operative on the fuel supplyv rate regulator to cause said regulator to change the controlled rate of fuel delivery by-an amount sumcient to maintain a given temperature in the furnace.

8. In a control system for furnaces provided with a source of air supply and a header having burners connected thereto to which a mixed fuel gas is delivered, a regulator for operating a member controlling the fuel supply rate and having a pressure responsive control element for the air supply regulator, a leak-off line connected to said second pipe for varying the pressures acting on the gas pressure responsive elements of said regulators, and means for adjusting the rate of leak-off to compensate for changes in the heating quality of the mixed gas so that the rates of air and fuel supply will be properly proportioned .for any value of heating quality 7. In a control system for furnaces provided with a source of air supply and a header having burners connected thereto to which a mixed fuel gas is delivered, a regulator for operating a member controlling the fuel supply rate and having a pressure responsive control element for controlling the regulator, a regulator for operating an element controlling the air supply rate and having a pressure responsive element connected with the air supply and operated in accordance with the rate of air delivery to the furnace, and a pressure element actuated by fuel gas pressure,

both said air and gas responsive elements operating to jointly control the air supply regulator to cause the air supply rate to vary by and in accordance with the gassupply rate for a given controlling the regulator, a regulator for operating an element controlling the airsupply rate and having a pressure responsive element connected with the air supply and operated in accordance with the rate of air delivery to the furnace, and a pressure element actuated by fuel gas pressure, both said air and gas responsive elements operating to jointly control the air supply regulator to cause the air supply rate to vary by and in accordance with the gas supply rate for a given heating quality of the mixed gas fuel, a pipe connected to the fuel gas supply header, an orifice in said pipe, a' second pipe connected through said orifice to said first men-' tioned pipe and to the pressure element of the fuel supply regulator and a pipe connected from. said second pipe to the gas pressure responsive element of the air supply regulator, a leak-off line connected to said second pipe for varying the pressures acting on the gas pressure responsive elements of said regulators, means .for

varying the rate of leak-off through said line,

JAMES K. MAWHA.

heating quality of the mixed gas fuel, a pipe 

